Proximity discovery is one of the main features of device to device (D2D) services. To facilitate proximity discovery, one device announces a discovery message or signal, and another device monitors for this discovery message or signal. In order for the monitoring device to discover the proximity of the announcing device, it must receive and decode the discovery message or signal. In order for this process to be successful, both announcing and monitoring devices must know which radio resources will be used to transmit the discovery signal or message. In a cellular network the allocation of radio resources for the transmission of the discovery signal/message is done by the network.
In 3GPP LTE Release 12, two types of discovery resource allocation are supported. In Type 1, announcing user equipment (UEs) autonomously select resources from a transmission (TX) discovery pool to transmit their discovery messages. The pool of possible discovery resources for Type 1 is allocated by the serving eNodeB (eNB). In Type 2, announcing UEs request, and are assigned, transmission resources by the serving eNB. For both of these types, a monitoring UE must wake up and monitor all discovery resources allocated by the eNB, regardless if there is a UE announcing a discovery message/signal in each particular resource. This procedure is energy inefficient, and wastes UE battery life.